Practice = get better. Improving those muscular groups doesn't necessarily improve your ability to do the handstand pushups.... of course it helps, but the best way to get better at something is by just doing it over and over. I max out around 5 handstand pushups... but I don't have the balance to do it without a wall like I used to when I did capoeira

I used to do these all the time. Practice them initially with your feet touching the wall for balance until you can crank out at least fifteen with good form. Then practice six inches or so away from the wall, using it only to prevent yourself from falling over. A few weeks of this and you should progress to the point of being able to do them without any balancing help.